In printing machines, particularly in sheet-fed offset printing machines, it is necessary, for purposes of quality control, to detect actual reflectance values by means of measuring devices preferably located inside the machine, and to derive therefrom quality data, or manipulate variables for process control. Particular use is made of densitometers/spectral densitometers, color-measuring instruments and so-called "spectrophotometers". With these "spectrophotometers", it is advantageous that the light, of a measuring point can be analyzed spectrally by means of these measuring instruments, and can be analyzed with respect to various criteria by further digital processing. Thus, for example, it is possible by means of digital weighting, to use the spectral reflectance of a measuring point to derive both the color density values which have long been used in printing technology, and colorimetric parameters in accordance with the spectral distribution curves of the CIE standard observer. The measured reflectance values described above (densitometric, colorimetric, and spectral) can be obtained both on measuring fields, which have been specially configured and expressly printed in the same forms, and at measuring points which can be arbitrarily selected on the subject (the actual image). The latter has the advantage that there is no need for additional space on the printing carrier and that, furthermore, the quality and process control data are derived directly from the actual printed product.
In the case of measuring devices which are used, in particular, to obtain spectral reflectance data directly inside the running machine, it is necessary to adapt the sensor sensitivity, the illuminance, and the measuring field size to one another. Specifically, in the case of rapidly running printing machines, for a given measuring field size (millimeter range) and a maximum achievable illuminance, only certain specific measuring times are available within which the sensor system, specifically of a spectral reflectance measuring instrument, has to convert the received light quantity. This means that the measuring spot geometry is a function of the aperture restrictor geometry, the relative speed between the printing carrier and the measuring field, the measuring instrument, and the integration time of the sensor(s). The effects of a change in the relative speed of the printing carrier and therefore the measuring field relative to the measuring instrument (printing speed) can be minimized within given limits by varying the integration time of the sensor system. However, this does not cover the entire range of print speed. In order to utilize fully the dynamic range of the sensor system, and thus to also achieve the selected accuracy, it is further necessary to change the illuminance. In the case of high print speeds, more light will have to be applied to a measuring field of prescribed size than in the case of lower print speeds.
EF 0 373 283 B1 discloses a measuring device for inspecting the quality of a printing carrier, where a neutral density filter, with a continuously changing optical density of transmission, is moved in the illuminating-beam path of the measuring field illumination as a function of the printing speed. The movement is accomplished by a remotely controlled drive, which obtains its signals from the controller. The controller is connected to a sensor system which detects the speed of the printing machine. As a result, this measuring device can be used to always scan measuring fields of given size in the optimum region of sensor sensitivity where the illuminating device has a constant control and the print speed varies. However, having a moveable neutral density filter with a controller is disadvantageous because of additional moving parts, sensitivity to mechanical disturbing influences, and exposure to possible contamination.
For the purpose of adapting a plurality of illuminating sources to the same color temperature, it is known from Wo 95/00335 A1 that it is possible to calibrate measurements to a calibration standard (white standard). In this process, the color temperature of the illuminating device of the respective measuring device, or a measure, reproducing the color temperature of the spectral energy distribution of the illuminating device is processed with the aid of the measured photometric variables. This procedure and devices are likewise used inside a printing press where the printing carrier has a relative speed with respect to the measuring device.
An illuminating device for a color measuring instrument in which additional measuring transducers are provided for detecting the radiation produced by a halogen lamp is known from DE 40 13 422 A1. The signals from the transducers are used to achieve a desired color temperature of the illuminating radiation by varying the lamp voltage.